This invention relates to a lithographic printing plate, more specifically to a lithographic printing plate using a silver complex diffusion transfer process.
With regard to a lithographic printing plate using a silver complex diffusion transfer process (the DTR method), some examples are described on pages 101 to 130 of Andre Rott and Edith Weyde, "Photographic Silver Halide Diffusion Processes", published by the Focal Press, London and New York (1972).
As described therein, there have been known two kinds of lithographic printing plates using the DTR process, i.e., a two sheet type in which a transfer material and an image-receiving material are separated and a mono-sheet type in which these materials are provided on one support. The two sheet type lithographic printing plate is described in detail in Japanese Provisional Patent Publication No. 158844/1982. Also, the mono-sheet type is described in detail in U.S. Pat. No. 3,728,114.
In a lithographic printing plate using paper as a support, a high quality printing including printing endurance is difficult due to elongation of the plate during printing or soaking of water. In order to solve these problems and to improve printing characteristics, a film support is used. As such a film support, there may be used, for example, a cellulose acetate film, a polyvinyl acetal film, a polystyrene film, a polypropylene film, a polyethylene terephthalate film, or a composite film in which a polyester, polypropylene or polystyrene film, etc. is/are covered by a polyethylene film. However, the lithographic printing plate using a film as a support is improved in elongation property of the printing plate or soaking of water, but there remains problems such as printing endurance, water retaining property, and further mounting property to a printing machine, etc.
Thus, in order to solve the various problems involved in the lithographic printing plate using paper or a film as mentioned above, there have been known a silver complex salt system lithographic printing plate using a metal, particularly an aluminum plate as the support. They are described in detail in Japanese Provisional Patent Publications No. 118244/1982, No. 158844/1982, No. 260491/1988, No. 116151/1991 and No. 282295/1992 and U.S. Pat. Nos. 4,567,131 and 5,427,889. In the above mono-sheet type lithographic printing plates using the silver complex diffusion transfer process which employ an aluminum plate as a support (hereinafter referred to as "an aluminum lithographic printing plate"), a physical development nuclei layer is provided on a grained and anodized aluminum support, and a silver halide emulsion layer is further provided thereon. A general process for making the lithographic printing plate comprises the steps of exposure, development processing, water washing processing (washing off: removal of a silver halide emulsion layer, hereinafter merely referred to as "washing processing"). Moreover, for protecting the lithographic printing surface, finishing processing is usually carried out.
Specifically, metallic silver image portions are formed on the physical development nuclei by development processing, and by washing processing subsequent thereto, a silver halide emulsion layer is removed to expose the metallic silver image portions (hereinafter referred to as "silver image portions") on an aluminum support. Simultaneously, the surface of the anodized aluminum itself is exposed as non-image portions. In the aluminum lithographic printing plate which is the object of the present invention, printing endurance is remarkably improved as compared with the silver salt printing plate using a paper support or a plastic support as mentioned above, but printing endurance of 150,000 sheets or more cannot yet be obtained stably and further improvement is desired.
On the other hand, in the aluminum support to be used in the present invention, graining and anodizing are carried out on the surface of the aluminum support in order to improve adhesiveness with the silver image portions formed on the surface thereof and to improve water retention property. In this roughening treatment (so-called graining), there are mechanical roughening treatments such as ball graining, wire graining, brushgraining, etc., chemical roughening treatments which are carried out by chemically dissolving aluminum with a chloride, a fluoride, etc., and electrolytic roughening treatments in which aluminum is electrochemically dissolved. Moreover, treatments such as desmutting treatment, post treatment, etc. are usually carried out.
With regard to surface treatment of an aluminum support for lithographic printing plate, a large number of roughened aluminum plates different in shape, structure, etc. of the roughened surface have been proposed depending on the purpose of printing. For example, there is disclosed an aluminum support in which the relationship between an amount of an anodized film and a surface roughness is specified in Japanese Provisional Patent Publication No. 260491/1988, and an aluminum support in which the surface shape is specified in Japanese Provisional Patent Publication No. 64293/1995. Also, there is disclosed that printing characteristics are markedly affected by a plateau (primary structure) and pit (secondary structure formed on the surface of the plateau) which are formed by using mechanical roughening and chemical etching as well as electrolytic roughening in combination in Japanese Provisional Patent Publication No. 28893/1981.
In the field of a PS plate (a pre-sensitized plate) using a photosensitive resin, various kinds of aluminum supports which are similarly roughened and anodized have been proposed and supplied from many manufacturers.
The surface shape of the above-mentioned aluminum support is determined by the combination of many conditions such as the conditions of the roughening treatments, chemical roughening treatments or electrolytic roughening treatments, the conditions of anodization, the alloy composition of the aluminum plate and the like, and it can be easily expected that printing characteristics such as ink receptivity, printing endurance, etc. are fluctuated depending on these various conditions. Particularly, the lithographic printing plate utilizing the silver complex diffusion transfer method which is an object of the present invention is easily affected by a subtle difference between the date of the manufacture (difference between lots) even when the aluminum supports prepared by the same preparation conditions are used. Accordingly, a lithographic printing plate using a silver complex diffusion transfer process which is not affected by the surface shape and the surface treatment of an aluminum plate and gives high printing characteristics stably has been earnestly desired.